Dear Prof Humbert and co-authors,

Thank you for submitting your replies to the reviewer reports. One reviewer requested minor revisions, with their key points being 1) the need to increase visibility of the data and methods, either as a dedicated section in the main text or better signposting to the Appendices, and 2) the suggestion to include an additional numerical perturbation experiment to assess the instantaneous impact of ice-shelf loss on the upstream flow. The 2nd reviewer indicated that major revisions are needed to warrant publication of your manuscript, with their main points being 1) the need for restructuring and/or expansion of most sections, in particular the Appendices, 2) missing information in figures and misalignment between figures and text, and 3) several ambiguities regarding terminology. First of all, I would like to echo the authors and extend my sincere thanks to both reviewers for their thorough reports, which contained lots of good suggestions on how to improve the manuscript. I would also like to thank the authors for their detailed replies, which -I think- comprehensively address the main concerns and all minor points. At this stage, I invite you to submit a revised version of the manuscript using tracked changes, if possible.

All the best,
Jan De Rydt - editor

Dear Prof Humbert and co-authors,

Thank you for your submission of a modified version of your manuscript entitled ‘Precursor of disintegration of Greenland’s largest floating ice tongue’. In line with the reviewer comments you made significant changes to the manuscript that have greatly improved its structure and clarity. I also welcome your addition of newly acquired data and appreciate the need to include further co-authors to acknowledge their contribution. Based on my own reading of the manuscript, I think further substantial editorial changes are required at this stage, to improve the readability of the manuscript, and, in some places, clarify the methodology and terminology. Please find a detailed list of suggested changes below.

At present, I believe the discussion section remains incomplete. In line with suggestions by the second reviewer, I would like to see ‘more nuanced reflections on the limitations of diagnostic simulations’. I have provided more details in the line-by-line comments below. Furthermore, I think a more in-depth discussion is needed on why the ‘calving style’ has changed: can you identify a robust causal relationship between changes in ice front location, pinning point ungrounding, and changes in rift dynamics? Would it be possible to present additional maps of the stress field, which cover both the era before and after the change in calving style? Can you detect any evidence in the stress field for a change in the preferred direction/mode of rift propagation, once the calving front migrated upstream of the pinning points? Such an analysis could provide a strong additional line of evidence to support your statements and conclusions about a distinct change in the direction of rift propagation (or ‘calving style’ as it is referred to in the manuscript), and address causality based on well-defined glaciological parameters and changes in geometry, rather than based on the rather hand-wavy comparison to other calving events elsewhere.

I look forward to receiving a point-by-point response to the comments below, and an updated manuscript with tracked changes.

All the best,
Jan De Rydt

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LINE-BY-LINE COMMENTS:

(all line numbers refer to the manuscript with tracked changes)

L1 replace ‘so far’ by the timeframe of available observations
L1-2 The first sentence of the abstract talks about changes to the ice tongue of 79N, whereas the second sentence compares this to changes to the grounded ice of ZI. This seems like comparing apples and oranges. The third sentence then jumps back to 79N. Is the comment about ZI needed here?
L4 ‘will’ lead to sea level rise, or ‘is likely’ to contribute to future sea level rise?
L5 employed
L6 add comma: …calving front, from…
L7 unclear what is meant by ‘normal’ tongue-type calving. Is this commonly used jargon?
L8 maybe replace ‘increase’ by ‘expansion’
L8 the description of the melange and ice bridge is difficult to comprehend without the help of Fig1 and 8. Could you reformulate to say that damage upstream of the main calving front has created areas of open water and an ice-berg melange?
L11 maybe say ‘future ice-front retreat and complete ice-shelf disintegration’ rather than ‘break-up or disintegration’, which sound identical.
L11 ‘discharge of grounded ice’ instead of ‘ice discharge’
L12 add …’south-eastern area of the ice shelf…’ for clarity
L26 ‘was taking place’ or ‘took place’?
L26 The reference to calving style might need some further context. For example, can you say that since the first observations in the 1980s, all calving events have followed a similar pattern, following the extension of a lateral rift over several years?
L31 ‘gone beyond’ in the upstream or downstream direction?
L42-45 by ‘different mechanisms’ do you mean a ‘variety of mechanisms’, or do you mean that crack formation is controlled by different mechanisms compared to crack propagation? In the latter case, I think references are needed, and a clear distinction needs to be made between what mechanisms are responsible for crack formation, and what mechanisms are responsible for crack propagation (or deformation).
L46 and following: some of these concepts are explored in detail in https://tc.copernicus.org/articles/14/1673/2020/
L64 …‘are’ stabilizing... I would prefer: ‘are buttressing’
L65 …‘an’ increase…
L66 ‘most recent’ instead of ‘latest’? Can you add a specific time instead of staying ‘in the past decade’?
L69 I think you use ‘stable’ to mean ‘unchanged’ throughout the manuscript. It might be good to clarify this somewhere, in order to avoid confusion with the term ‘stability’, which comes from dynamical systems theory and means something different: a steady system is stable if it returns to the same steady state following a small perturbation.
L71 ‘an’ ice discharge increase
L72 Why do you start the sentence with ‘However’, if the measurements and modelling results are consistent? You will also need to include Hill et al. 2018 (https://doi.org/10.5194/tc-12-3907-2018) here.
L73-82 These sentences sound rather fragmented. Can you reformat into a coherent paragraph?
L80 ‘mandatory’ is a strong use of words; perhaps say ‘informative’ or ‘a first step towards’
L81-81 Do you have evidence from the published literature (or even anecdotal evidence) for this statement?
L83 what do you mean by ‘catastrophic’? I would remove this part of the sentence. Also, by ‘change in load situation’, do you mean ‘change in ice-shelf geometry’?
L88 Do you mean ‘a moderate retreat of the ice front’ instead of ‘a moderate calving rate’? Pine Island is one of the fasted flowing Antarctic glaciers, and even a steady ice front location would require the glacier to have one of the highest calving rates on Earth.
L92 ‘Set this into the context of the stress regime’: can you be more precise? Are you linking changes in calving rate / calving flux / calving extent / … to changes in principle deviatoric stress / …?
L94 replace ‘destabilisation’ with ‘further retreat’
L95 ‘numerical perturbation experiments’ instead of ‘simulations’.
L117 I think you can already refer to IR1 and IR2 in line 112
L129 ‘ALS elevation data ARE lacking…’
L131-132 An ‘alteration between grounding and ungrounding’: this sentence confused me, because earlier on you say that this area has ungrounded. Do you mean ‘episodic’ grounding and ungrounding?
L131-134 I would fold this into the discussion above, where you provide evidence from other sources. I don’t see why the radar data should be treated separately.
L137 ‘…result from two processes’ instead of ‘…from two instances’
L139 Unless I’m looking at the wrong feature in Fig4 (distance along track ~2.4km) it appears to me that the difference in surface elevation is about 7 or 8 meters?
L142 You suggest surface melting as a process. How about dynamic thinning?
L146 ‘In addition’ instead of ‘Next’
L156 ‘…is similar in summer 2022,…’
L166 when you refer to the stresses in the floating tongue, say that this will be discussed later.
L176 How did you estimate components of the stress tensor? From ISSM?
L176-180 Can you show the evolution of the stress field as the calving front & rifts develop between the 1990s and 2020s? So reproduce Fig 6 but for different snapshots in time? You might find evidence in the stress field that could help explain the changes in rift propagation direction (or ‘calving style’ as it is referred to in the paper) after 2014. The following work might be of interest in that context: https://tc.copernicus.org/articles/13/2771/2019/.
L183 The propagation into an area with lower stresses might well lead to the arrest of the rift, although the behaviour also depends on the spatially heterogeneous material properties (ice thickness, critical stress intensity etc.) so I think a more nuanced statement is needed here.
L182-184 Inferences about the type of fracture are provided as facts, but some explanation that links back to the explanation of the different modes of fracture in L46 etc. would make it easier to understand this paragraph. E.g. why is crack D ‘clearly’ a mode I crack?
L185 ‘…along SURFACE rivers or lakes…’
L198-205 I find this paragraph hard to follow. Do you infer evidence for tidal bending at the location of rift E from the density of fringes in the interferogram? If so, I would expect there to be tidal bending -and hence ‘fatigue cracks’ - in many other locations with a similarly strong phase gradient in the interferogram. Can you explain this in more detail in the manuscript please?
L206 pre-existing
L218 ‘We further consider THE potential future evolution OF the calving front’
L219 ‘will detach’ instead of ‘detaching’
L234 ‘was last observed’ instead of ‘disappeared’
L236 ‘area’ instead of width
L238 What do you mean by a ‘normal’ calving front?
L241 remove ‘from south’ before the reference to the figure.
L242 ‘A’ bulging zone. Also refer to the relevant figure here.
Fig1 caption. In the description of panel b, refer to IR1 & 2 when you talk about the pinning points. Replace ‘marked in blue’ by ‘shaded in blue’
Fig3 caption. Is ‘The location of the profile is presented in Fig 1d’ needed here?
Fig5 caption. Can you indicate the location of the radar profiles?
Fig6 caption. ‘scale bar’ instead of ‘scale bare’
L267 ‘larger’ instead of ‘more dramatic’
L276 ‘predominantly’ instead of ‘in the majority’
L282 is 0.8% correct here?
L284 ‘significant increase’ or ‘166% increase’ instead of ‘massive’
L288 ‘First we discuss the WIS, which has …’
L292 ‘…15% of area…’: do you mean the area of the ice bridge?
L289-294 The comparison based on broad-brush similarities in the geographical setting for both ice tongues seems rather speculative, given the detailed stress arguments that are invoked in the remainder of the manuscript. Is there any strong evidence from the literature that ice tongue area and other geographical descriptors such as lateral confinement, are good metrics to determine the vulnerability to collapse?
L295 ‘retreated past A lateral embayment’. Also: it is not clear to me what is meant by a lateral embayment. Can you describe the geographical setting in more detail?
L307-310 Again, this sounds rather speculative. Do you have any evidence that the stress field changed significantly between calving events, i.e. once the ice front retreated beyond the pinning points?
L300 ‘…far easier to retreat past this area’: Do you have a reference to the published literature for this statement?
L320-325 The discussion about air and ocean temperatures is rather detached from the remainder of the discussion and reads like an afterthought. Can this be integrated better, e.g. by saying that you consider the different external drivers that might have contributed to the changes observed at the ice front, and then systematically discuss the different possibilities?
L329 ‘…crack E is none of these but ‘COULD BE’ a tidal fatigue crack…’
L330 ‘speculate’ instead of ‘interpret’
L334 ‘long’ instead of ‘large’
L339 The use of terminology ‘transition from stability to instability’ is misleading in the context of dynamical system theory, since no stability analysis was performed in this paper. Please consider replacing this statement with ‘is potentially at the onset of a major retreat phase’ or similar.

General comment about the discussion section: I’d like to echo Reviewer 2 and say I’m missing a more in-depth discussion about the model results. Besides the changes in GL flux, it would be interesting to explore the underlying reasons for the modelled behaviour. For example: if I understand correctly, your model is initialized with velocities prior to the formation/propagation of some of the rifts. Since you simulate a 5% increase in GL flux as a result of calving following the formation of the rifts, I wonder if that is consistent with observations? Can you also provide some further intuition about the changes in the buttressing parameters, e.g. what is the reason for the large areas of unbuttressed ice shelf in the calv2fjord experiment? Would one not expect shear contact with the lateral mountain slopes to provide some amount of buttressing?

In L102 you state that “Based on this variety of datasets we aim to investigate whether recent changes of the 79NG configuration are indicating a regime change, what exactly is the cause of the changes and how this will impact the stability of 79NG.” yet I find little direct evidence in the text for the ‘cause of the changes’. Are you referring to the ungrounding of the pinning point? If so, I think stress maps at different times before/after the ungrounding might help you identify how the glaciological conditions have changed, and how this has impacted on the preferred direction of fracture propagation.

Dear prof Humbert and co-authors,

Thank you for taking the time to formulate detailed replies to my comments, and for implementing the changes in the manuscript. I think the text is close to being ready for publication. To assist with the final stages of the editorial process, can I please ask you to proofread the manuscript one final time, as I could detect numerous typos in the newly reworded/added paragraphs. Please also add a link to the data in the "Code and data availability" section, in particular for the DEMs, radargrams and model output. Once you have uploaded the corrected manuscript, your work will be forwarded for publication. I would like to thank you for choosing The Cryosphere for publishing your work, and I would like to extend a sincere thanks to both reviewers for their support.

All the best,
Jan De Rydt - editor